Early prime movers or power sources such as the mill stream water wheel; and, later the steam engine, created a need for transferring power from one place to another through torque transfer. An early example of torque transfer was the belt and pulley assembly, frequently utilized in machine shops to drive various machines.
The early prime movers generated low revolutions per minute (rpm) and mechanically simple torque transfer mechanisms were capable of creating very little power. In portable machines, chains and sprockets are often used to transfer power. The most common example of this form of torque transfer is the ordinary bicycle. The bicycle sprocket and chain are a roller cam assembly; and the relative motion between the roller in the chain and the sprocket is that of an involute. It is the shape of the sprocket that enables the power to be transferred smoothly.
With the need to transfer torque between adjacent parallel and perpendicular shafts, something else had to be devised. In fact, the old water wheels that were used to grind grain employed wooden pegs as gear teeth to transfer torque; but, as rpm and torque increased, it was observed that a fluctuation in speeds occurred, creating destructive forces. This fluctuation resulted from the variations during rotation of the effective radii between the meshing of the pegged wheels.
The variation of the effective radii was a result of the shape of the pegs. The first solution was to develop a double cycloid tooth with a convex shape above the pitch circle and a concave shape below the pitch circle. These gears looked something like a plurality of rollers evenly spaced with their axes parallel to the input shaft and attached to the surface of a wheel. If two concave arcs were placed near the point of attachment and the surface of the wheel, it would create a tooth that resembled the teeth used in these gears. This design solved the velocity fluctuations. However, the gears were difficult to manufacture and their shape made them highly susceptible to failure.
In the latter part of the nineteenth century, the involute gear was developed. It was both much stronger and cheaper to make. In calculations of gear strength, the teeth were treated as a series of cantilevered beams, and an engineer named Lewis devised a table of values called the Lewis Form Factor that simplified calculations of torque capacity of spur gears. This table is used in calculations today.
1. Field of the Invention
The present invention utilizes the parameters of existing spur gears such as a choice of pitch and pitch circle in a meshing “roller ring-toothed cam” configuration that employs roller action as opposed to the high-friction sliding action of spur gear. In the present invention, it is the trochoidal shape that eliminates the interference between meshing members. This configuration permits fewer, larger and stronger teeth in the smaller meshing member and allows the design of smaller, stronger and more compact gear trains. Fewer, larger teeth are possible because the shape of the cam tooth is determined by the relative motion between rotating bodies.
A roller ring-toothed cam combination can be designed with external meshing members, internal meshing members, bevel gear arrangements and rack and pinion combinations. In all of these proposed assemblies, either the larger or the smaller member can be the roller ring or the toothed cam. The point to be made here is that any of the roller ring-toothed cam assemblies disclosed herein will permit the design of compact, high-strength gear trains with very high train values and pure rolling contact.
2. Description of the Prior Art
In U.S. Pat. No. Re. 17,811 issued Sep. 23, 1930, the inventor identified the point on the cam shape where the shape changes from concave to convex.
He noted that this point on the curve would carry the load, caused by the contact of the roller for a longer period of time. To overcome this dwell problem, he utilized a prolate trochoid curve where the generating point is three-fourths the distance from the center of the generating circle compared to the radius of the generating circle. This distance must also equal the eccentricity of the crank. The dwell point is a point on the cam that has a zero radius of curvature arid this point cannot support loads without deteriorating the cam. For this reason, cycloidal curves should not be used in roller cams. By locating the generating point so far from the surface of the generating pitch circle, the effective component of the forces between the roller and the cam is greatly reduced. thereby severely limiting the capacity of this prior art mechanism to handle torque. U.S. Pat. No. 5,247,847 dated Sep. 28, 1993 is particularly relevant to the instant disclosure. It describes the use of roller cams for external assemblies where the roller ring is always smaller, in a rack and pinion assembly where the roller ring is the pinion; and in a torque assembly where the roller cam is in a bevel gear design. The definition and description of the roller cams in this reference utilize a mathematical presentation with the line of centers being held constant and the two rotating members being rotated. This procedure follows the historical graphical procedure using conjugate curves. Furthermore, this procedure can develop the same shaped tooth that is created by the use of epicycloidal mathematics described herein. In the latter case, one of the pitch circles is held stationary while the line of centers and the other pitch circle, with the generating point, is allowed to rotate. In both cases, the tooth shapes generated are identical. The practical problem with these epicycloidal and cycloidal tooth shapes is that there is one point on the face of each tooth that has a zero-radius of curvature. And this point will not handle any forces without inflicting serious deterioration on the cam surface.
Two additional references that are somewhat relevant to the disclosed invention include U.S. Pat. No. 3,783,712 dated Jan. 8, 1974, which discloses a roller cam assembly employing a long arcing slot containing two suspended rollers that are separated by a wedge to fill the slot and transmit torque; and U.S. Pat. No. 4,604,916 dated Aug. 12, 1986, which uses a free floating set of rollers on a retaining ring as used in roller bearings. This ring of rollers has one more roller than the inner member has notches and one less roller than the outer member has notches and all rollers are in contact with both the inner member and the outer member at all times.